In video recording systems, it is known to modulate the frequency of a luminance carrier in accordance with a luminance signal separated from a composite video signal, thereby to generate a frequency-modulated (FM) luma signal. It is further known to translate the frequencies of a chrominance signal separated from a composite video signal, which frequencies are from the upper portions of the spectrum of the composite video signal to lower frequencies, to generate a "color-under" signal that can be frequency-multiplexed below the frequency-modulated luminance carrier to form a video signal for recording. Such a frequency translation or down-conversion procedure for chrominance signal implements what is known in the recording art as a color-under recording system.
In a color-under recording system, the signal reproduced during playback is supplied to frequency-selective filters for separation into an upper-band frequency-modulated luminance carrier and a lower-band color-under signal. The luminance (Y) signal, in accordance with which the frequency of the luminance carrier was modulated during recording, is detected. The frequencies of the color-under signal are translated back to their original spectrum during playback in an up-conversion procedure to restore chrominance (C) signal with its the in-phase (I) and quadrature (Q) components. The restored chrominance (C) signal can be recombined with the detected luminance (Y) signal to regenerate the composite video signal, which can then be used to modulate a low-level picture carrier to be supplied together with a low-level sound carrier to a television receiver. Or, in apparatus combining a video tape machine and a television receiver, the chrominance (C) signal can be supplied directly to synchronous detectors for detecting color-difference signals therefrom for application to color matrix circuitry, and the detected luminance signal can be supplied directly to a luma delay line preceding the color matrix circuitry.
As is well known, in video cassette recorders (VCRs) according to the standard VHS format, the input video chrominance (color difference) information which is modulated on a chroma sub-carrier (nominally 3.58 MHz for NTSC composite video) is separated from the luminance information prior to recording and then frequency down-converted by heterodyning against a 4.21 MHz carrier to provide a sub-carrier at approximately 629 KHz (40 times the horizontal scanning frequency) with the sidebands reversed in order from the NTSC chroma component, to be directly recorded onto the videotape in the frequency spectrum below the recorded luminance component (i.e., below approximately 1.2 MHz.) as the so-called "color-under" sub-carrier signal. As is also well known, in order to reduce beat and cross-talk between mutually adjacent recorded tracks, during the recording process the phase of the VHS format's 629 KHz color-under carrier is shifted 90.degree. each line in every track scan (i.e., field), so that its phase is shifted or rotated +90.degree. per line (advanced) on odd tracks and -90.degree. per line (retarded) on even tracks. See for example U.S. Pat. Nos. 3,723,638 to Fuhita, 4,068,257 to Hirota et al. and 4,178,606 to Hirota, the disclosures of which patents are incorporated hereinto by reference thereto.
In U.S. Pat. No. 5,113,262, issued 12 May 1992 to inventors of the invention described herein and to others, entitled VIDEO SIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK and incorporated by reference into this specification, a video signal recording system is described that modifies VHS video cassette recording by folding the spectrum of a 5 MHz bandwidth luminance signal to frequency-modulate the luminance carrier, the modulation sidebands of which are higher in frequency than an accompanying color-under signal. A motion (M) signal is derived descriptive of the frame-to-frame changes in the luminance signals, is used for controlling adaptive filtering of the luminance signal during recording, and is recorded for controlling adaptive filtering of the luminance signal during playback.
The video signal recording system described in U.S. Pat. No. 5,113,262 includes an adaptive deemphasis circuit in the record path for luminance signal and an adaptive reemphasis circuit in the playback path for luminance signal. The adaptive deemphasis circuit includes circuitry for detecting the level of the high-frequency portion of the luminance signal and includes circuitry for variably reducing the level of the high-frequency portion in response to the detected signal level. If the level of the high-frequency portion of the luminance signal is high, then the level of the high-frequency portion is reduced by a maximum amount; if the level is low, then the level is reduced by a minimum amount. The adaptive reemphasis circuit in the playback path performs substantially the inverse operation. This adaptive reemphasis circuit includes circuitry for detecting the level of the high-frequency portion of the unfolded luminance signal, and circuitry for variably increasing the level of the high-frequency portion in response to the detected level. If the level of the high-frequency portion of the unfolded luminance signal is relatively high, then the level is boosted by the maximum amount; if the level is relatively low, then the level is boosted by the minimum amount.
U.S. patent application Ser. No. 07/787,690 filed 4 Nov. 4, 1991 by C. H. Strolle et alii, entitled SYSTEM FOR RECORDING AND REPRODUCING A WIDE BANDWIDTH VIDEO SIGNAL VIA A NARROW BANDWIDTH SYSTEM describes improvements in the deemphasis and reemphasis functions in a video signal recording system of the type described in U.S. Pat. No. 5,113,262. U.S. patent application Ser.No. 07/787,690 also describes improvements in the recording of the motion (M) signal, in which the motion (M) signal is used to modulate the amplitude of a 252 kHz second under carrier located below the 629 kHz color-under carrier in the frequency multiplex signal generated for recording on video tape. U.S. patent application Ser.No. 07/787,690 describes the use of quadrature selective filters (QSFs) for separating the overlapping amplitude-modulation sidebands of the color-under and second carriers when playing back from the video tape. (The specification and drawing of U.S. patent application Ser.No. 07/787,690 are included herein, by inclusion in an appendix at the close of this specification.)
The correspondence between the deemphasis and reemphasis functions in a video signal recording system of the type described in U.S. Pat. No. 5,113,262 can be adversely affected by the presence of noise in the signal recovered from the video tape. U.S. patent application Ser.No. 08/008,813 filed 25 Jan. 1993 by C. H. Strolle et alii and entitled ADAPTIVE DEEMPHASIS AND REEMPHASIS OF HIGH FREQUENCIES IN VIDEO TAPE RECORDING, UTILIZING A RECORDED CONTROL SIGNAL describes a control signal (G) used for controlling the deemphasis function during recording being recorded on the video tape, to be reproduced during playback for controlling the reemphasis function. U.S. patent application Ser.No. 08/008,813 is a continuation-in-part of U.S. patent application Ser.No. 07/604,494 filed 26 Oct. 1990 and indicates that it is preferable to multiplex the control signal into the modified VHS video signal described in U.S. Pat. No. 5,113,262 to avoid the need for further heads for recording and playback of the control signal. (The specification and drawing of U.S. patent application Ser.No. 08/008,813 are are included herein, by inclusion in an appendix at the close of this specification.)
There is, then, a need in the color video cassette recording art to convey in a manner compatible with already established video recording procedures auxiliary signals, such as the motion signal (M) and control signal (G) described above, which provide for improved regeneration of the color television signals. In accordance with the invention the color-under signal is augmented with another under signal carrying the motion signal (M) and control signal (G) described above (or, alternatively, other additional information such as digital audio signals). If this second under signal carries additional information of significant bandwidth, the frequency band in which it reposes will have to overlap the frequency band of the color-under signal at least to some extent. The problem, then, is how to encode the color-under and second under signal such that they can be separated from each other after sharing the same frequency band during recording.
Luma/chroma separators using line-comb filters are known by designers of television receivers and designers of video recorders. In one type of luma/chroma separator, a composite video signal and that signal as delayed by the interval of one horizontal scan line can be differentially combined to separate the chrominance signal from the luminance signal, and the same signals can be additively combined to separate the luminance signal from the chrominance signal. This is because both the luminance and chrominance signals are highly correlated from line to line; but the 3.58 MHz color subcarrier, being an odd multiple of half scan line frequency, is out-of-phase at corresponding positions along current scan lines in the two differentially combined composite video signals differentially delayed by one scan line. In an alternative known type of luma/chroma separator, a composite video signal, that signal as delayed by the interval of one horizontal scan line, and that signal as delayed by the interval of one horizontal scan line are combined in (1/4):(1/2):(1/4) ratio, to separate the luminance signal from the chrominance signal, and are combined in (-1/4):(1/2):(-1/4) ratio, to separate the chrominance signal from the luminance signal.
A baseband signal, like the luminance signal in the composite video signal, is not a suitable signal for recording on a magnetic tape, particularly when using helical scanning techniques as usually done in video recording. A modulated carrier is required during recording for coupling from the record amplifier through the rotary transformer to the heads on the headwheel rim and for coupling from the head to induce appreciable magnetic flux variations in the magnetic tape. A modulated carrier is also required during playback so the magnetic flux variations in the magnetic tape induce appreciable electrical playback response from the heads and for coupling from the heads on the headwheel rim back through the rotary transformer to the playback amplifier.
Suppose that a modulated carrier that is an even multiple of half scan line frequency is combined with a modulated carrier that is an odd multiple of half scan line frequency and the combined signal is down-converted by heterodyning with an unmodulated carrier that is an odd multiple of half scan line frequency, thereafter to be frequency multiplexed with an FM luma signal. Suppose further that during playback the down-converted signal is recovered and up-converted to regenerate the combined signal. The carrier that is an even multiple of half scan line frequency is in-phase at corresponding positions along current scan lines in the two differentially combined composite video signals differentially delayed by one scan line. The carrier that is an odd multiple of half scan line frequency is out-of-phase at corresponding positions along current scan lines in the two differentially combined composite video signals differentially delayed by one scan line. Accordingly, the inventors note, line-comb filtering can be used to separate the carrier that is an even multiple of half scan line frequency from the carrier that is an odd multiple of half scan line frequency. The carrier that is an odd multiple of half scan line frequency can be the color subcarrier encoding I and Q signals in what are essentially quadrature amplitude-modulation (QAM) sidebands per the prior art. The carrier that is an even multiple of half scan line frequency can encode the M and G signals in QAM sidebands.
Alternatively, suppose that a modulated carrier that is a first odd multiple of half scan line frequency is combined with a modulated carrier that is a second odd multiple of half scan line frequency and the combined signal is down-converted to be frequency multiplexed with an FM luma signal. Suppose further that during playback the down-converted signal is recovered and up-converted to regenerate the combined signal. The carrier that is a first odd multiple of half scan line frequency can be the color subcarrier encoding I and Q signals in what are essentially quadrature amplitude-modulation (QAM) sidebands, and the carrier that is a second odd multiple of half scan line frequency can encode the M and G signals in QAM sidebands. Line-comb filtering can be used to separate the carrier that is a second odd multiple of half scan line frequency from the carrier that is a first odd multiple of half scan line frequency, the inventors point out, if the carrier that is a second odd multiple of half scan line frequency is made to be in-phase at corresponding positions in the two differentially combined composite video signals differentially delayed by one scan line. This can be accomplished by reversing the phase of the carrier that is a second odd multiple of half scan line frequency from each horizontal scan line to the next, the inventors point out.